It is known that combustion of fossil fuels, such as petroleum, natural gas or coal, produces nitrogen oxides (NO.sub.x) which get into the atmosphere and which are harmful to the environment. The nitrogen oxides are regarded to be co-responsible for forest damage through "acid rain" as well as for the formation of "photochemical smog." Accordingly, there is great interest in reducing the NO.sub.x content of combustion gases. Flue gases are understood to include both those from furnace installations and those from internal combustion engines.
It has long been known that nitrogen oxides can be transformed into N.sub.2 and H.sub.2 O selectively by NH.sub.3, i.e. in the presence of an excess of oxygen. Also, several catalysts for this reaction are already known.